To start preparations for CAT 2018 you need to first focus on mastering the basics of each section.

For quantitative ability, you can start preparing through Arun Sharma book which has a good explanation of the theory and question solutions. Also, it provides a high variety of questions with the different difficulty level.

For Verbal ability, you should start reading lots of good novels written by good authors like Charles Dickens, Agatha Christie, Sydney Sheldon etc. This will help you in improving your English Literature. For improving your vocabulary you should start memorizing 3 words daily with their meaning and correct pronunciation. You should start using these words in your daily life so that you can memorize them in a better way. For Reading Comprehension section you should start reading different passages in different newspaper, magazines etc. and understand the thought process of the author. You need to see what actually he wants to convey through his passage, to who is he targeting and his way of communication. This will help you in solving the maximum questions asked below the passage. You should also see the Arun Sharma book to check out and practice the different types of questions asked in the exam.

Along with this, you should start giving lots of online mock test and analyse solutions of each one of the thoroughly which will help you in overcoming your mistakes and come up with a strategy to definitely improve your marks and percentile. It will also help you to know about your strength and weakness which will help you in attempting easy questions first, that will reduce your chance of committing a silly mistake in an easy question or missing out an easy question. Along with this, you will also get enough time to attempt medium level difficult questions which will overall increase your speed with accuracy which leads to increase your percentile. 

Testfunda, Oluveboard, hitbullseye, cracku.in, youth4work.com, bschool.careers360.com are some of the sites where you can get online mock test papers for all CAT, SNAP and XAT exam

All the best for your preparations!!